The present invention can be applied to all those cases where the presence of closely adhering coatings on copper substrates is required, such as applications on components subjected to thermal cycles, for example in the fields of power generation and in foundries.
An example of a possible application are those components that, in nuclear reactors, face onto the plasma. In these cases the presence of a coating is necessary to provide resistance against heat and erosion by ion bombardment. In nuclear plants, many components (for example the divertor), which are made for example of copper to guarantee effective heat exchange levels, are subjected to the erosive action of particles and ions from the plasma, as well as being subjected to the high temperatures resulting from the presence of plasma. As a result of this, their average lifespan is reduced to unacceptable levels, and problems also arise in relation to pollution of the plasma and of the atmosphere in the reactor itself by the particles and compounds that evaporate, sublimate or are removed by sputtering. Furthermore, the presence on the surfaces of metals such as cobalt or nickel results in the formation of radioactive isotopes, with the consequent danger of pollution and damage to human health.
The materials currently considered most suitable to protecting the surfaces of said components against erosion are the following: tungsten and boron carbide (materials with a high and a low atomic number, respectively). In particular, tungsten is considered to be the most interesting material. However, application of tungsten is not easy, as this material is solid up to 3750 K and characterised by a thermal expansion coefficient that differs considerably with respect to that of copper.
The methods used to apply a protective coating of said material are currently those of welding or brazing of solid tungsten tiles and plasma-spray coating. However, each of these methods involve problems that make it difficult to obtain reliable coatings, in particular for applications that foresee thermal stress during service, due for example to the thermal cycles induced by the operations for start-up and shut-down of the plant.
Brazing and electron-beam welding make it possible to apply solid tungsten tiles, even tens of millimetres in thickness, on the components to be protected. However, the use of this method involves the following problems:
the difficulty in producing tungsten tiles of complex shapes, capable of providing an exact reproduction of complex curved surfaces; PA1 the difficulty in welding/brazing on surfaces with a complex shape; PA1 the difficulty in welding/brazing on very large surfaces; PA1 the action into the component of stresses induced by the welding temperature, due to the different expansion coefficients in the substrate and in the tungsten and to the high temperatures required to join the two elements. PA1 stresses induced by the deposition temperature due among other things to the high levels of power required to melt the particles of tungsten; PA1 low adhesion of the coating, in the specific case of copper alloy components, due to the speed with which a layer of copper oxide, which is fragile and difficult to remove, is formed on the surface of the copper in the presence of oxygen in the atmosphere. PA1 surface activation of the copper product or alloys thereof; PA1 optional heat stabilisation to a temperature below the temperature causing deterioration of the chemical and physical properties of the copper or alloys thereof; PA1 deposition of a first layer, for example of Ni or alloys thereof; PA1 optional interdiffusion heat treatment; PA1 optional surface activation of the first layer; PA1 thermal stabilisation of said product at a temperature of between 20.degree. and 400.degree. C.; PA1 deposition of one or more intermediate layers of at least one of the following: Al; AlSi; Cu; Ni; NiAl; NiCr; NiCu; MCrAlY (wherein M can be Ni, Co, Fe or mixtures thereof), mixtures thereof, or mixtures of said intermediate layers and said thick coating; PA1 thermal stabilisation of the product coated with said intermediate layers at a temperature of between 20.degree. and 300.degree. C.; and PA1 deposition of said thick coating on the product coated with said intermediate layers.
Mechanical application of plates, locked for example using screws, does not appear to be practicable, due to the imperfect adherence between the plate and the substrate, which results in a drastic reduction in the thermal conductivity of the whole component.
On the contrary, thermal spraying makes it possible to coat even objects with a complex geometry. It has the advantage of allowing local repairs to be carried out. However, at the present time the use of thermal spraying is limited by the following problems:
The problem of low adhesion appears to be the most critical for function of the coating, and the most important to solve, as the reliability of the component when in service depends upon it, in particular during operations for start-up or shut-down of the plant.
This problem is emphasized by the presence of thermo-mechanical stresses induced in the coating at every change in temperature, due to the considerable difference in thermal expansion coefficients (close to 20.times.10.sup.-6 K.sup.-1 for copper; approximately 4.times.10.sup.-6 K.sup.-1 for tungsten), this stress being concentrated in particular in the copper/tungsten interface area.
The difference in expansion coefficients is also the cause of creation within the coating of so-called residual deposition stress (see for example, T. W. Clyne, S. C. Gill: "Residual stress in thermal spray coatings and their effect on interfacial adhesion."--Journal of Thermal Spray Technology, March 1995). This stress, when added to the stress generated by the temperature variations to which the component is subjected during function, result in a considerable decrease in the adhesion of the coating itself. Limitation of the maximum thickness obtainable with plasma-deposit coatings is mainly due to the phenomenon described above (see, for example, T. Shinoda et al.: "Development of dry process for heavy thickness coating layer"--Acts of the International Thermal Spray Conference '95, Kobe, J, which indicates a maximum thickness obtainable with plasma-deposit coatings of around one millimeter).
In order to obtain thick and reliable coatings, it is therefore necessary to solve the problem of adhesion of the coating itself to the copper substrate, and of distribution of residual stress to the tungsten-copper interface.
From the above considerations, it can be seen that, at the present state of the art, it is practically impossible to obtain thick and reliable coatings on copper and alloys thereof using thermal spraying technology. The statement made above for the specific case of thick tungsten coatings is also true in general, and the more so the more the thermal expansion coefficients of the component and coating materials differ.